The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as defined by the Jun. 20, 1990 directive 90/385/CEE of the European Communities Council, more particularly to pacemaker, defibrillators and/or cardiovertors which are capable of delivering low-energy stimulation pulses to the heart for the treatment of the cardiac rate disorders, and even more particularly to the adjustment of the stimulation pulse amplitude (voltage level) over time.
The voltage level (amplitude) used to stimulate the cardiac cavities (ventricular or atrial) of a human is a value typically selected in a range between 1.5 and 5.0 V (xc2x110%), and which is adjustable in 0.25 V steps or increments within that range. The amplitude must be sufficiently high to cause the depolarization of the cavity myocardium (i.e., a stimulated heart beat). It is, however, important to avoid amplitudes that are too high to spare the lifespan of the battery, because the stimulation energy applied to the myocardium and, therefore, the corresponding consumption of the implant device, is proportional to the square of the amplitude (and also to the duration) of the stimulation pulse delivered.
Moreover, the capture threshold, i.e., the minimum energy level needed to cause a contraction of the myocardium, is a value which can vary over time. Thus, it is desirable to be able to reevaluate at regular intervals the stimulation amplitude level needed by operating a test of the threshold of the effectiveness of the stimulation, called the xe2x80x9ccapture threshold testxe2x80x9d or more simply xe2x80x9ccapture testxe2x80x9d. The capture test thus determines the minimum amplitude needed to cause a stimulated heart beat.
The amplitude of the stimulation pulse is then adjusted on the basis of the capture threshold measured, and typically is a value that is twice the value of the measured capture threshold, subject to a minimum level (typically 1.5 V) and a maximum level (typically 5.0 V) amplitude. This provides a degree of confidence that the amplitude delivered will exceed the capture threshold and cause a beat.
International application WO-A-93/02741 and its corresponding U.S. Pat. No. 5,411,533 (commonly assigned herewith to Ela Medical S.A.) describe an automatic algorithm for testing the ventricular capture threshold, which is used in the Talent(copyright) model pacemaker marketed by Ela Medical. A clinical follow-up of patients equipped with this device revealed that, in certain cases, the algorithm of automatic capture threshold test will be deceived and give inaccurate measures, probably because of fusions (stimulations intervening in a concomitant way to a spontaneous QRS (ventricular) event) occurring at the time of the capture test.
These anomalies result in an overvaluation of the capture threshold compared to the real capture threshold of the patient. Because reevaluation of the capture threshold (and possible readjustment of the stimulation amplitude) is typically performed every six hours, an excessive amplitude level which is maintained for these six hours, although not in itself dangerous, is a source of overconsumption of battery energy, and thus leads to a reduction of the lifespan of the implant device.
It is, therefore, an object of the present invention to overcome the aforementioned disadvantage by proposing various improvements to the algorithm for the capture threshold test, so as to detect these anomalies which are likely to erratically occur, and to neutralize their effects when they occur.
Broadly, the present invention is directed to the type of implant device described by the above mentioned U.S. Pat. No. 5,411,533, which includes a stimulation means for delivering to the heart stimulation pulses of low energy for the treatment of the cardiac rate disorders, these stimulation pulses having a predetermined amplitude and a predetermined duration, and means for adjusting the amplitude of the stimulation pulses, comprising means for automatically measuring the capture threshold at predetermined intervals, and delivering a measured capture threshold value which can then be used as a basis for the adjustment of the stimulation amplitude.
According to the invention, the means for adjusting the amplitude also comprises means for validating the capture threshold value, suitable to operate a coherence test as between the last measured capture threshold value and a reference value which is a function of at least one of the corresponding capture threshold values measured previously. If the coherence test result is positive (i.e., incoherence exists), the adjustment of the stimulation pulse amplitude level is inhibited. In a preferred embodiment, the aforementioned reference value is one of the previously measured capture threshold values.
Preferably, the validating means produces a positive result of the coherence test when the difference between the last and the penultimate measured capture threshold values is higher than a first predetermined difference, and when the difference between the penultimate and the antepenultimate measured capture threshold values is less than a second predetermined difference. The aforementioned first or second difference is preferably equal to one step of the measurement of the capture threshold level, or an integer multiple of this step (i.e., an integer multiple for n an integer, nxe2x89xa71). Further, the first predetermined difference is preferably greater than or equal to the second predetermined difference.
In another preferred embodiment, in the event of a positive result with the coherence test, the validating means starts a reiteration of the measurement of the capture threshold, the stimulation amplitude level being preferably adjusted after the reiteration on the basis of the lowest of the two capture thresholds thus measured, before and after the reiteration. In this last case, the device also includes means, responsive to the value of the capture threshold measured after the reiteration being higher than the one measured before the reiteration, for reducing the duration of the atrio-ventricular delay (xe2x80x9cAVDxe2x80x9d), and setting of the automatic measuring means of the capture threshold with this new value of the AVD.
The device also includes means, responsive to the automatic measuring means giving an indicator indicating the impossibility of delivering a result, to reduce the duration of the stimulation pulses, and setting of the automatic measuring means with this new value of the pulse duration.